Various types of transponder systems are well known in the prior art but for the most part such systems utilizing a radar interrogator do not permit voice communications between the interrogator and the transponder. Other prior art systems use radio responder beacons for measuring the altitude of an aircraft. Of course, the use of pulse width modulation in voice communication systems is old but such prior art systems make no suggestion of application in transponder systems. Still other prior art systems use radiosonde for obtaining meteorological data but do not use voice communications.
The present invention overcomes the deficiencies of the prior art by providing a voice-modulated transponder system including an interrogator utilizing pulse repetition frequency (PRF) modulation and a transponder utilizing pulse width modulation (PWM) which can both locate the position of the transponder and simultaneous therewith permit two-way voice communication between the interrogator and the transponder. In the described embodiment, wherein the range of the system is under 10 miles, the interrogator is basically a small X-band secondary radar set which is airborne and may be mounted on a helicopter or light aircraft. The interrogator is compact and light enough to be carried on light aircraft or helicopters and at the same time may be added conveniently to larger tactical aircraft without great difficulty. The transponder which is hand carried is characterized by its extreme compactness, lightness and low battery drain. The transponder is compatible with most existing airborne radars in that it could reply as a beacon to their interrogations even though voice communications was not possible. This compatibility feature fixes the interrogation frequency of the system at X-band and the reply frequency at 9310 MHz. The system has capabilities for obtaining a range and bearing of the transponder with high accuracy to within a small fraction of a mile and the range of the system is very flexible depending on the particular applications. Both the airborne interrogator and the transponder are omnidirectional so that there is no need to scan for each to find the other. The present invention may have military application for communication between ground personnel and aircraft and could also be used for aircraft navigation and search and rescue operations on both land and sea.